Evaluating contribution of soil water to paddy rice by stable isotopes of hydrogen and oxygen

Rice is a highly water-consumptive crop under frequent flooding. However, the contributions of soil water and ponded water to rice are unclear, complicating efforts to water management in paddy ecosystem efficiently. Using stable isotopes of the oxygen (delta O-18) and hydrogen (delta D), we aimed to determine isotopic compositions of ponded water, soil water, and groundwater, and then to evaluate their contributions to rice crop. Two types of paddy water management, one under continuous flooding (CF) and the other under alternate flooding and drying (AFD), were investigated. The isotope ratios of rice stem water, ponded water, soil water along soil profile of 0-50 cm, irrigation water, groundwater, and rain water were determined by isotope ratio infrared spectroscopy. The direct inference method and Iso-source model approach showed that rice appeared to use soil water from < 10 cm depth, which accounted for approximately 80 % of rice water in the CF field, while in the AFD paddy soil, > 90 % of rice water was 0-15 cm from soil depth. The contribution of ponded water to rice crop was negligible (< 1.1 %). Water use efficiency under the AFD condition was significantly greater than that under CF condition. This study provides direct evidence that water can be saved considerably if the ponded water is reduced.